1. Field of the Invention
The invention relates to a pump for delivering a liquid, which pump is in particular suitable for delivering a liquid additive into the exhaust line apparatus of an internal combustion engine.
2. Related Art
For the purification of the exhaust gases of internal combustion engines, exhaust-gas treatment devices are known into which a liquid additive is supplied for the purposes of purifying the exhaust gases. An exhaust-gas purification method implemented in such exhaust-gas treatment devices is the selective catalytic reduction (SCR) method in which nitrogen oxide compounds in the exhaust gas are reduced with the aid of a reducing agent. Ammonia in particular is used as reducing agent. Ammonia is often stored in the motor vehicle not directly but in the form of a liquid additive that constitutes a precursor solution of the reducing agent. The liquid additive may be converted to form reducing agent within the exhaust gas (in the exhaust-gas treatment device) or outside the exhaust gas in a reactor provided specifically for the purpose. For the SCR method, urea-water solution is used as liquid additive. A 32.5% urea-water solution is available under the trade name AdBlue®.
For the delivery of liquid additive out of a tank and for the dosed supply of the liquid additive to the exhaust line apparatus, at least one pump is provided. A pump of this type should be as inexpensive and reliable as possible. It is particularly advantageous if the pump can also perform a dosing (metering) function, that is to say delivers a predefined amount of the liquid additive very accurately. Furthermore, during the delivery, the least possible pressure fluctuations should be generated in the liquid additive, because these can adversely affect the spray pattern of a nozzle for atomizing the liquid additive in the exhaust-gas treatment device. A further requirement is that the pump should be as quiet as possible.
A further important aspect in the case of pumps for delivering liquid additives is that the liquid additives for exhaust-gas purification that are used can freeze at low temperatures. The urea-water solution mentioned above, for example, freezes at −11° C. In the automotive field, such low temperatures can arise for example during long standstill periods in winter, wherein the liquid additive expands as it freezes. The pump should accordingly also be constructed such that it is not damaged by freezing liquid additive.